In signal-processing systems having two partially correlated data sets with relative spectral errors, phase-related problems may arise. Such is the case with interferometric synthetic aperture radar (IFSAR) systems operating in a bistatic mode. In a bistatic configuration, only one antenna of a pair transmits, but both antennas receive simultaneously. This is in contrast to monostatic IFSAR operation, wherein both antennas transmit and receive essentially independently on alternate pulses. In a monostatic mode, the same local oscillator (LO) is used for both the transmit and receive functions. As such, the LO phase noise which degrades the SAR output is the result of a first difference operation on the phase noise with a time offset equal to the roundtrip range delay. This provides significant cancellation for the low-frequency components of the LO phase noise (common-mode cancellation).
In a bistatic mode of operation, however, the transmit and receive functions may use independent local oscillators. Accordingly, there is no phase noise cancellation in the bistatic channel of such an interferometric system. In fact, the two noise sources add, and the phase noise requirements on both LOs are much more severe. (see, for example, J. Autennan, "Phase Stability Requirements for a Bistatic SAR", Proceedings of the IEEE National Radar Conference, March 1994).
These LO errors become phase errors in the demodulated SAR data. These, in turn, contribute to a loss of resolution, high sidelobes and smearing of the image data. In an IFSAR system, LO errors also cause loss of correlation in the interferometer comparison. The phase information contained in the IFSAR correlation is normally converted to elevation map data via several additional processing steps. Due to the loss of correlation, however, the phase will be noisy, and the elevation data accuracy will also be degraded.
In a bistatic mode of operation, extreme stability or some other compensation technique is required to mitigate IFSAR bistatic phase errors. In terms of stability, one approach is to use high-quality atomic clocks; another approach is to actively lock the two oscillators together. These solutions could have potentially significant cost impact, particularly in a spaceborne system. With high-quality oscillators, the problems with bistatic operation typically causes low-frequency phase errors and high-azimuth sidelobes in the bistatic image channel for modest aperture times (i.e., several seconds). Autofocus approaches, both quadratic and higher-order, can help to alleviate this problem, but the degree of compensation is only partial, and still requires high-quality clock stability. In addition, this technique may not be reliable in all terrain clutter environments.